Why are rising CO2 levels caused by burning fossil fuels?

How fast are carbon dioxide levels in the atmosphere rising? How can you tell that rising carbon dioxide levels are caused by burning fossil fuels and not natural causes such as bushfires, volcanoes, rising ocean or soil temperatures?

By Genelle Weule

Levels of CO2 in the atmosphere have significantly risen since the industrial revolution. (Source: Vikter Korotayev/Reuters)

How fast are carbon dioxide levels in the atmosphere rising? How can you tell that rising carbon dioxide levels are caused by burning fossil fuels and not natural causes such as bushfires, volcanoes, rising ocean or soil temperatures?

"Cape Grim is always lower than Mauna Loa because there is a transfer of CO2 between the Northern Hemisphere, where the CO2 is released and the Southern Hemisphere," explains Fraser.

While the amount of CO2 varies from season to season — higher in winter and lower in summer — the overall trend in is a continual rise of around 2 ppm of CO2 every year, he says.

"Everything points to the fact that the changes we're seeing are due to the burning of fossil fuels," says Fraser.

Around 85 per cent of global CO2 emissions from anthropogenic sources come from burning fossil fuels, nearly 10 per cent from forest deforestation and the rest from cement production and other industrial processes.

Not all carbon is equal

Carbon is released into the atmosphere from a variety of sources including respiration of plants and microbes, bushfires, volcanoes, and burning fossil fuels. Some of these mechanisms have a distinct carbon isotopic signature.

There are three types of carbon isotopes — molecules of carbon with different atomic weights: carbon-12 (12C), carbon-13 (13C) and carbon-14 (14C).

Carbon-12 is the most common form found in nature. Carbon-13 is about 1 per cent of the total. Carbon-14, which is produced in the upper atmosphere, accounts for an even much smaller amount.

Plants take up all forms of carbon in the process of photosynthesis, but they prefer the lighter carbon-12, leaving carbon-13 behind in the atmosphere.

"That enriches carbon-13 in the atmosphere compared to carbon-13 in the biosphere, which is why carbon-13 is more abundant in the atmosphere," says Fraser.

But measurements of the composition of CO2 show a decline in the ratio of carbon-13 to carbon-12 in the atmosphere of about 3 per cent per year, says Fraser.

"The decline in carbon-13 is consistent with the carbon-13 composition of fossil fuels that are released to the atmosphere when they're burnt," says Fraser.

So how do we know that rising CO2 levels are caused by burning fossil fuels and not phenomena such as bushfires?

Fossil fuels — coal, oil and gas — are made out of ancient plants and microorganisms — so they are also depleted in carbon-13. The key difference is that, unlike living plant material, fossil fuels contain no carbon-14.

"Carbon-14 decays in the atmosphere at a known half life [of around 5,700] years, therefore fossil fuels, which are millions of years old, contain no carbon-14," explains Fraser.

Oxygen levels

Another clue that the rise in CO2 is caused by burning fossil fuels and not the natural biosphere cycle is a drop of atmospheric oxygen that parallels the rise in carbon.

Burning of fossil fuels involves the consumption of oxygen.

"[Plants] uptake oxygen in photosynthesis and release it in respiration so they don't alter the oxygen balance in a stable biosphere," says Fraser.

"As you burn fossil fuels and create carbon dioxide you actually remove an equivalent amount of oxygen from the atmosphere," says Fraser.

"Oxygen levels are going down at about 4 ppm per year, a similar rate at which carbon is going up in the atmosphere, after allowing for the carbon that's taken up by the ocean and the Earth's vegetation. And that's what you'd expect for a combustion process."

However, as oxygen makes up around 21 per cent of the atmosphere, the amount of oxygen that's removed does not pose an environmental threat, says Fraser.

The three graphs above taken from the 2012 Snapshot of the Climate report show: a) increasing concentrations of CO2 in the atmosphere in parts per million; b) decreasing 13C/12C ratio (δ13CO2 — which is derived by comparing the ratio of 13C/12C in the atmosphere to the 13C/12C ratio in an international standard (limestone), minus 1, multiplied by 1000, to give a per mille reading); and c) decreasing oxygen to nitrogen ratio from data gathered at Cape Grim and the Scripps Institute of Oceanography.

What about volcanoes?

A decline in the 13C/12 C ratio in the atmosphere is also a good indicator that volcanic eruptions do not contribute to the long-term trend, says Fraser.

"Carbon that is released from volcanoes has… higher levels of carbon-13 than it does with carbon that's released from the burning of fossil fuels or the respiration of plants," he says.

If volcanic eruptions played a part we wouldn't see the steady decline of carbon-13 levels or the steady increase in CO2. In fact, we don't even see spikes of activity, except for the most extreme volcanic events such as Pinatubo in 1991, he says.

"If volcanoes were a significant source of carbon dioxide then you'd have a big spike in the growth rate corresponding to when the volcano goes off. We don't see that, it's just a steady increase."

"The amount of carbon that volcanoes emit is just not enough.

"People think that volcanoes must be releasing large amounts of carbon but they're not, relative to the amount of carbon that's in the atmosphere. And hence for example when we saw the big eruption last year in South America, we saw no impact of that on our carbon record at Cape Grim at all.

"None whatsoever, it didn't release enough carbon to impact our carbon record as much as for example the biosphere does which generates small seasonal bumps, or fossil fuel release which generates the long-term trend," says Fraser.